Magnetic resonance imaging (MRI is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the z, y or x axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonance frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
MR images may be created by applying currents to the gradient and RF coils according to known algorithms called “pulse sequences.” The selection of a pulse sequence determines the relative appearance of different tissue types in the resultant images. Various properties of tissue may be used to create images with a desirable contrast between different tissues. Many specific techniques have been developed to acquire MR images for a variety of applications. In order to reduce the time required to perform MR examinations, pulse sequences have been developed that permit extremely rapid acquisitions of a large amount of data. Time reductions are particularly important for acquiring high resolution images as well as for suppressing motion effects and reducing the discomfort of patients in the examination process. Dramatic reductions in acquisition time have been obtained through a technique referred to as echo planar imaging (EPI). In EPI, a bi-polar gradient waveform is used concurrently with the echo-train acquisition. Each echo in the echo train in individually phase-encoded, usually by a small blip gradient pulse, to produce a line of k-space data. In doing so, multiple k-space lines can be acquired in a single excitation. The k-space lines acquired in a single excitation can be used to reconstruct an image, a technique known as “single shot EPI.”
In certain clinical applications, it is desirable to acquire “diffusion weighted” images in which tissues that have either higher or lower water self-diffusion characteristics relative to other tissues are emphasized. Typically, diffusion weighting is implemented using a pair of large gradient pulses bracketing a refocusing RF pulse. The diffusion weighting gradient sensitizes the MR signals to the diffusion of water molecules which can be subsequently used as a contrast mechanism to distinguish different tissues. Diffusion weighted imaging has been combined with EPI techniques (DW-EPI) and many clinical diffusion weighted imaging applications are performed using a single-shot sequence, such as single-shot echo-planar imaging (ss-EPI). For example, diffusion weighted imaging (DWI) is being increasingly adopted in routine clinical MR scans to assess the development or degeneration of tissue microstructures.
Reduced field of view (rFOV) approaches have been developed as a way to achieve single shot echo-planar (ss-EPI) diffusion weighted imaging (DWI) with acceptable imaging distortion in susceptibility-prone anatomies and as an enabler for high resolution DWI. A two-dimensional (2D) spatially selective echo-planer (EP) RF excitation pulse with blipped gradients along the slice select axis may be used to achieve reduced field of view by exciting a limited extent in the phase field-of-view direction. Such an rFOV excitation can also be beneficial for other sequences with extended echo trains such as fast spin echo. However, one challenge with such an approach has been the limited slice coverage provided per acquisition. Concerns about, for example, partial saturation in slice locations that may overlap with the periodic side lobe locations of the RF excitation profile limit slice coverage to the number of slices that can be accommodated within the distance between the periodic side lobes of the RF excitation profile. In many applications, such as axial DWI of the spine or breast, this does not provide sufficient coverage.
It would be desirable to provide a system and method for reduced field of view MR imaging that increases the slice coverage by using a multiband RF refocusing pulse. Reconstruction techniques such as parallel imaging may be applied for resolving signal from simultaneously acquired slices utilizing the coil sensitivity differences between the simultaneously acquired slice locations.